Traditional exhaust gas recirculation systems (hereinafter referred to as “EGR systems”) recirculate part of the exhaust gas emitted from engines to intake systems in order to improve the exhaust performance. The EGR systems mix the part of the exhaust gas with intake air to decrease the combustion temperature of the mixed air, thereby reducing the emission of nitrogen oxides (NOx). The EGR systems include paths connecting exhaust systems and intake systems of the engines, and valves for adjusting flow rates (hereinafter referred to as “EGR quantities”) of exhaust gas in the paths. The openings of the valves are controlled by controllers.
Some engines being practically used are equipped with turbochargers including turbines that are rotated by the energy of exhaust gas and that drive compressors provided coaxially thereto to supercharge compressed air to cylinders, in order to increase the output torque of the engines. In recent years, variable geometry turbochargers have been widespread that include flow-rate adjusting mechanisms called variable vanes in the vicinity of the turbines of the turbochargers. The variable geometry turbocharger is a supercharger of which the supercharging pressure can be controlled through adjusting the opening of the variable vanes and thus controlling the flow rate of exhaust gas onto the turbine. In an engine equipped with such a variable geometry turbocharger, a feedback control of the pressure (hereinafter referred to as “intake manifold pressure”) in an intake manifold of the engine is carried out by adjusting the opening of the variable vanes in response to the operating state to keep the intake manifold pressure at a target pressure.
The EGR systems are categorized into two types, i.e., a high-pressure EGR system for recirculating exhaust gas from a position in the exhaust system upstream of the turbine to a position in the intake system downstream of the compressor, and a low-pressure EGR system for recirculating exhaust gas from a position in the exhaust system downstream of the turbine to a position in the intake system upstream of the compressor. An EGR system called “dual-loop EGR system” includes both the high-pressure and low-pressure EGR systems.